Ctrl+click to pick colors immediately

A keyboard shortcut is now available for picking colors from
the canvas more quickly.  Hold either (left or right) [Ctrl] key
while clicking, and the color selector option will appear.
Release the mouse button over a color on the canvas to choose it
(or outside the canvas to abort).
Closes https://sourceforge.net/p/tuxpaint/feature-requests/209/

Also,
 * Don't play 'bubble' paint sound when color picker or selector
   are aborted (e.g., via their "Back" buttons).
 * Replace many instances of "NUM_COLOR - 1" and "NUM_COLOR - 2"
   (which correspond to color chose via picker (palette) and
   selector (canvas), respectively) with #define's that
   represent them. (Avoid magic numbers.)

docs/CHANGES.txt

@@ -70,10 +70,16 @@ http://www.tuxpaint.org/
     * Show a "pipette"-shaped mouse pointer when selecting a
       color from the color palette, or the picture.
 
-    * WIP Provide a keyboard shortcut for picking colors from
-      the canvas more quickly.
+    * A keyboard shortcut is now available for picking colors from
+      the canvas more quickly.  Hold either (left or right) [Ctrl] key
+      while clicking, and the color selector option will appear.
+      Release the mouse button over a color on the canvas to choose it
+      (or outside the canvas to abort).
       Closes https://sourceforge.net/p/tuxpaint/feature-requests/209/
 
+    * Don't play 'bubble' paint sound when color picker or selector
+      are aborted (e.g., via their "Back" buttons).
+
   * Localization Updates:
     ---------------------
     * Albanian translation